The present invention relates generally to a method and apparatus for producing ethanol from alcoholic mash produced by fermentation and which contains between about 1.5% and about 15% by volume of alcohol. The method comprises the steps of distillation, rectification, purification and dehydration. The method is carried out in individual stages each in a relatively simple, energy-saving and cost reducing manner.
The prior art, as evidenced by German Pat. No. 615,885, relies on processes in which some or all of the vapor discharged from a distillation column is conveyed to a dehydrating column. A portion of the vapor may also be directed from the distillation column to a rectification column. Such prior art processes may, however, only be used for processing slightly alcoholic mash. When it is necessary to process stronger mash, the vapor withdrawn from the distillation column is not sufficient to heat the dehydration column and, as such, it is not effective for such use.
An alternative method for the production of alcohol from fermented mash is described in Austrian Pat. No. 117,027, wherein the mash distillate is converted into either unconcentrated or concentrated alcohol in a distillation column. The alcohol is, thereupon, rectified into purified, concentrated alcohol in a rectification column followed by dehydration in a dehydration column. In order to reduce the amount of steam required to dehydrate the alcohol, the distillation column may be connected to the dehydration column via a conduit which heats the dehydration column using the vapor generated by the distillation column. The system is not particularly effective in conserving energy during distillation and rectification.
It is also known, as evidenced by German Auslegeschrift No. 11 65 549, to effect rectification of an alcohol-water mixture in a number of successive columns connected in series and to convey the vapors which are withdrawn from the head of the first column into the sump of the next column. The additional columns are heated in a similar manner.
Thus, it is already known in the field of distillation to heat a column by the heated vapor conveyed from another column.
Heretofore, while most operating plants were capable of only producing one type of alcohol at a time, it was also known, as described in Austrian Pat. No. 184,540, to simultaneously produce, in a single plant, an absolutized water-free alcohol and a purified aziotropic alcohol from a fermented, alcoholic mash. In accordance with the methods described in this patent, the unconcentrated alcohol issuing from the distillation column is processed into pure alcohol in a group of rectification columns or, as disclosed therein, into rectified alcohol. A portion of such alcohol is then conveyed through a cleaning or purification column and pure alcohol is withdrawn from such column. In accordance with this method, the distillation and rectification columns for preliminary or secondary cleaning or purification are each provided with discrete heating systems. In order to effect heating, the preliminary dehydration column is heated with vapor conveyed from the distillation column. The condensate from the purification column is used to heat the rectification column. Such apparatus can simultaneously produce both types of alcohol. However, it is not possible to alternately produce either of the two types of alcohol.
So long as thermal energy was inexpensive, distillation plants were built in such a way that the requisite heat energy was supplied to individual columns and withdrawn therefrom by an associated condenser. As a result of the recent increased cost for thermal energy, manufacturers of new plants have now reverted to the aforedescribed energy-saving systems for distillation plants with multiple utilization of the vapor. The technology which was applied for such purpose has led, for example, in the case of rectification plants, to at least a 50% reduction in the amount of thermal energy required.
This, however, has resulted in the necessity for constructing distillation columns for superatmospheric or subatmospheric pressures in order to achieve the requisite pressure drop or pressure gradient through the required number of columns which had to be connected with one another. See, for example, German Pat. No. 615,885.
In such cases, it was necessary to first determine whether or not the customary qualities were also achievable in the superatmospheric or subatmospheric range in order to produce ethanol. After testing, it was determined that the quality was the same.
In ethanol producing plants using the above techniques, it was heretofore necessary to use approximately 4.5 to 5 kg of steam per liter of pure alcohol, particularly where the alcoholic mashes to be distilled have an alcohol content of approximately 9% by volume.